In order to have power measurement and control, transmitters are configured with power control feedback loops responsive to power detectors. In common configurations for high (e.g., microwave) frequency bands, the power level is measured by a detector in a waveguide which is connected between the output of the power amplifier and the load.
In general, waveguides are used for transporting high frequency signals, in part because of their low-loss characteristics and ability to handle high power. Waveguide components are configured in a number of geometries, examples of which include ‘parallel’ with a pair of plates, ‘co-planar’ with a thin slot in the ground plane of one side of a dielectric substrate with or without a conductor in the slot, ‘dielectric’ with a dielectric ridge on a conductor substrate, ‘ridge’ with conducting ridges on the top and/or bottom walls, and ‘rectangular’ with a parallel-piped structure of a substantially rectangular cross section. Thus, although the discussion here examines rectangular waveguides, other waveguide may be suitable for power measurement.
One approach to power measurement can be described as the single probe approach, as shown in FIG. 1. The waveguide component is defined by its top, bottom, input and load planes, 12a–d, respectively. The waveguide has a single slot 14 in the bottom plane and a single probe 15 for measuring the power level protrudes into the waveguide through this slot 14. The probe 15 is often made of a conductive material and the potential generated thereby drives the detector diode 16. The output of the detector diode 16 is connected to a buffer amplifier 18 in order to isolate the detector diode from downstream components (not shown) and prevent their interference with its signal integrity.
As shown, forward signals traverse the waveguide from the input plane 12c to the load plane 12d. Ideally, there would be a perfect impedance match between the waveguide and the load (antenna or test equipment not shown) and the entire signal energy would be transferred from the waveguide to the load. In reality, however, the match is imperfect and results in reflections of the forward signals from the load plane 12d. The opposite-traveling reflected signals interfere with the forward signals and this produces a new wave pattern known as standing waves, which is what the probe 15 ultimately measures.
The amplitude of the standing waves is affected by the degree of interference of the reflected signals with the forward signals which is based on the degree of mismatch between the waveguide and the load. Then, because with the single probe configuration there is no isolation from the load mismatch, this measurement is strongly influenced by variations in the load conditions.
A second approach, described as a directional waveguide coupler, attempts to solve the problems associated with the unreliable power measurement inherent in the single probe configuration. FIG. 2 illustrates the directional waveguide coupler.
The directional waveguide 21 is designed for a particular frequency band with top, bottom, input and load planes 22a–d, respectively, and with the slots 24a and 24b in the bottom plane 22b spaced apart a quarter wavelength (or 90°). Attached to the bottom plane of the waveguide and facing the slots 24a and 24b is a coupler 23, also configured as a waveguide. The coupler 23 has a waveguide termination plate 26 and a bottom plate 25 with a slot 28 through which the power probe 29 protrudes. As before, the power probe 29 is connected to a detector diode 32 which is, in turn, connected to the buffer amplifier 34 to produce the detector output while isolating it from downstream stages.
Under ideal load conditions there would be a perfect match between the waveguide and the load (antenna or test equipment not shown), and the load plane 22d would transfer the forward signals from the waveguide to the load without losses. In reality, the load conditions are not perfect because of the load-waveguide impedance mismatch and the load plane 22d reflects the forward signals. The reflected waves interfere with the forward waves and whenever two waves of similar frequency travel in a medium in opposite directions standing waves are formed. Thus, the load plane acts as a constructive or destructive reflector based on its position relative to the resultant standing waves cycle. The same applies to the signals passing to the coupler through the slots 24a and 24b. 
The forward signals that pass through slots 24a and 24b, respectively, converge at the probe 29 in phase. This is because the forward signals moving through the waveguide 21 and slot 24b and those moving through slot 24a and the coupler 23 travel the same respective quarter wavelength (90°) distance. At the same time, reflected signals which pass through slot 24a travel the quarter wavelength (90°) distance twice, once in the direction toward slot 24a and once in the opposite direction toward the probe 29. In other words, reflected signals that pass through slot 24a are 180° out of phase relative to the reflected signals that pass through slot 24b. 
It is noted that a full cycle of the wave is comparable to a full circle of 360°, and any fraction of the circle in degrees is comparable to a fraction of the wave cycle which is the phase. When the forward and reflected signals are in phase (0° or 360° phase difference), the interference is constructive and produces a standing wave which is the sum of both (with twice the amplitude); and the interference is destructive when they are out of phase from each other. The phase shift (P) between the opposite-traveling waves can be 0<P<360°, where a 180° phase shift results in mutual cancellation of these waves.
Thus, the reflected signals converge at the probe 29 at 180° out of phase and cancel each other. Ideally, the probe 29 reads the magnified forward signals and none of the reflected signals. In reality, however, there is an imperfect match at the waveguide termination plate 26 and some of the reflected signals do end up converging at the probe with less or more than 180° phase shift.
The reason for this imperfection is any inaccuracy in the complex mechanical structure of the waveguide and coupler. Indeed, any variation in the operating frequency and/or the mechanical dimensions or material of the waveguide and coupler components can create a mismatch and, as a result, introduce some of the reflected waves at the probe 29. In particular, the frequency dependent waveguide termination plate design calls for different types of material to achieve the desired performance. Moreover, manufacture of the waveguide and coupler involves non-flexible frequency-dependent mechanical and electrical design for achieving performance such as isolation and power coupling. The two-part directional waveguide structure is hard to build and is even harder to replicate in commercial quantities.